This invention relates to a production of powder metal preforms and in particular to a reduction sintering method of as-atomized ferrous metal powders.
There are a number of different methods by which metal powders useful in the production of powder metal compacts, have been produced. These methods include, for example, electrolytic processes, ore reduction processes and water atomization processes. The latter process has recently come to the forefront, especially in the production of ferrous metal powders, since the process is generally more economical and produces particles of a shape and density which provide a powder compact with enhanced physical properties. When producing finely divided ferrous metal particles by water atomization, oxide films are formed on the surface thereof due to the reaction with oxygen. In order to produce powders useful for further compacting, the atomized powders must first be annealed in a reducing atmosphere such as hydrogen to soften the powders and reduce the oxide surface thereof.
The reduction process is generally conducted in a hydrogen gas current atmosphere at a reduction temperature of 1000.degree. C. to 1200.degree. C. for a time sufficient to reduce the surface oxide.
This reduction process requires a lot of hydrogen gas to be continuously supplied since dew point in the furnace rises as H.sub.2 O is formed by the reaction with hydrogen gas. In the reduction furnace using hydrogen, there also involves a possibility of explosion. Therefore a great care must be taken in operation to avoid a possible explosion.
As a result of this reduction procedure, the finely divided particles tend to agglomerate and form a cake-like structure, thereby necessitating an additional grinding stage to brake up the cake and finally achieve the desired particle shape and size distibutions required for further compacting.
Oxides containing Ni and/or Mo can easily be reduced, however, oxides containing Mn and/or Cr can hardly be reduced by conventional reduction or annealing procedures since the oxides containing Mn and/or Cr have relatively low free energies.